In typical power and free conveyors, a load carrier is mounted on a load trolley or trolleys which run on a load track. The load carrier is moved by a continuously running power or drive chain by engagement between a drive or pusher dog on the drive chain and a retractable trolley dog on the load carrier or trolley. The drive chain is supported by power trolleys running along a power track. In overhead power and free conveyors, a load supported by the load carrier is suspended below the tracks supporting the load trolleys and the drive chain. The pusher dog extends downward to engage an upwardly extending trolley dog. The trolley dog may be retracted from the pusher dog to allow the load to coast on a downhill section of the load track, to halt the load for operations thereon, or the like. On many power and free conveyors, the load trolleys incorporate accumulation mechanisms which cause drive disengagement of carriers approaching behind a halted carrier to prevent collisions between the carriers and loads.
Inverted power and free conveyors are similar to overhead power and free conveyors except that, as their name suggests, they are turned upside down. On inverted power and free conveyors, a power track supporting the power trolleys carrying the drive chain is at the lowest level. Above the power track is the load track supporting the load trolleys, with the load carrier above the load track. The pusher dogs of inverted power and free conveyors extend upward to engage downwardly extending trolley dogs which may be retracted to disengage drive from the load for the same reasons as for overhead track conveyors. Both overhead and inverted power and free conveyors find application in factories, such as on automotive assembly lines to carry automotive bodies as manufacturing operations are performed.
Drive chains for both types of power and free conveyors are similar and many types are formed by alternating center links and pairs of side links. In a typical drive chain, each center link is a vertically open endless loop with straight sides and cylindrical or semi-annular ends. The side links are elongated members or bars which are horizontally flattened. The opposite ends of an upper and a lower side link of each pair overlap the adjacent center links and are connected to the center links by cylindrical chain pins. The chain pins have T-shaped heads at opposite ends which engage recesses on the outer surfaces of the side links. The sides of the center links are "waisted" or dumbbell shaped and the side links have longitudinal slots in their ends to allow the drive chain can be assembled without tools. The center links pivot about the chain pins or the side links with chain pins pivot about the center links for the drive chain to pass about a curve having a vertical axis.
The drive chain is supported by the power track on power trolleys which are usually attached to center links at selected spacings. On overhead conveyors the power trolleys extend above the drive chain while on inverted conveyors, the power trolleys extend below the drive chain. The drive chain is usually driven by a large diameter drive wheel having gear teeth or sprockets which mesh between the side links to engage a trailing end of a center link.
Pusher dogs for such chains are usually in the form of side link pusher dogs which replace one of the side links of a pair. A side link pusher dog for an overhead conveyor has an integral dog or projection extending downwardly from a side link bar to engage an upwardly extending trolley dog. Conversely, a side link pusher dog for an inverted conveyor extends upward. Typically, side link pusher dogs have an integral chain pin extending from the side link bar on the opposite side from the dog projection. Thus, a side link pusher dog connects with a leading center link by the integral chain pin and with a trailing center link by a conventional chain pin. A conventional side link is then connected with the opposite ends of the integral chain pin and the conventional chain pin to complete the modified section of the chain.
In order to reduce frictional wear of the components of the drive chain and to prevent binding therebetween which might result in breakage, the frictionally engaging surfaces must be lubricated. This is usually accomplished by an automatic chain pin oiler machine which might also be adapted to lubricate power trolley wheel bearings. The oiler machine detects the presence of a center link and directs a shot of lubricant toward the bearing surfaces of the ends of the center link and the chain pins. The shot of lubricant is usually directed toward the slots in the upper side links on each side of the center link, and gravity and flexure of the pivotal joint disperse the lubricant over the bearing surfaces.
On a drive chain for an overhead power and free conveyor drive chain, a side link pusher dog is located on the lower side of a side link pair with a conventional slotted side link on the upper side of the pair. Thus, a conventional oiler machine can be used to lubricate such a chain since the pivot joints are accessible to the oiler nozzles through the slots on the upper side link. However, a problem arises in lubricating drive chains for inverted power and free conveyors, because the side link pusher dog is located on the upper side of a side link pair. While the trailing end of the side link pusher dog is slotted, the dog projection occupies the position at the leading end of the side link bar where a chain pin slot of an ordinary side link would be. Thus, there is no access for lubricating the leading pivot joint of a side link pusher dog for an inverted power and free conveyor drive chain using a conventionally configured chain pin oiler machine.
When pusher dogs of power and free conveyors engage the trolley dogs of stationary carriers supporting heavy loads, they have a tendency to react in a vertical direction away from the trolley dogs. Thus, in an inverted conveyor, the pusher dog tends to react downward. In order to avoid possible slippage of the pusher dog past the trolley dog during such reactions, the pusher dog is supported by power trolleys positioned close to the pusher dog. On a conveyor chain with side link pusher dogs, a power trolley, identical to the chain support trolleys, is usually attached to the center links immediately in front of and behind a side link pusher dog assembly. While it would be possible to provide a single power trolley on the conventional side link member of such an assembly, such a trolley would have to be specially designed since the chain support trolleys have a height designed for their attachment to the center links and would misposition a conventional side link pusher dog assembly. Thus, two trolleys per dog are required by the design of side link pusher dogs whereas only one per dog is functionally required. With a pusher dog every few feet, this can amount to a considerable expense on a long conveyor chain, which may run to several thousand feet in a large installation.
When it is necessary to replace a side link pusher dog because of wear or damage, the conveyor chain must be loosened and disassembled to free the old member. For this, it is necessary to move the side link type members toward the centers of the adjacent center links, which are occupied by the power trolleys. Thus, the leading and trailing power trolleys adjacent the pusher dog to be replaced must be removed to disassemble the chain.